Problem: William has taken 6 quizzes and his average score so far is 85. If he gets 100, a perfect score, on the remaining 4 quizzes, what will his new average be?
Explanation: If he gets 100 on the remaining quizzes, the sum of his scores will be $6 \cdot 85 + 4 \cdot 100 = 910$ His overall average will then be $910 \div 10 = 91$.